Method of reproducing information with equalization coefficient for the reproduced information

ABSTRACT

A method of reproducing information by scanning information marks, disposed with a predetermined mark pitch therebetween and recorded on tracks inside a predetermined information recording region, by an optical spot. The method includes detecting the size of the optical change resulting from the information mark, calculating a plurality of equalization coefficients used for an equalization processing for the size of the optical change detected for each of the information marks, and reducing the inter-symbol interference on the basis of the equalization coefficients by the equalization processing. The equalization coefficient used for the equalization processing of the size of the optical change by a first information mark is greater than said equalization coefficient used for the equalization processing of the size of said optical change by a second information mark which is longer than the first information mark.

[0001] This is a continuation application of application Ser. No.09/478,343, filed Jan. 6, 2000, and is related to application Ser. No.10/074,049, filed Feb. 14, 2002, which is a divisional of U.S.application Ser. No. 09/478,343, the subject matter of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to an information reproducing method forreproducing information recorded on an optical recording medium by usinga laser beam, and to an apparatus for executing this method.

[0004] 2. Description of the Related Art

[0005] In an optical information recording/reproducing apparatus forreproducing information marks recorded on information tracks of anoptical recording medium using a laser beam, the laser beam is condensedas small as possible on the optical recording medium by using anobjective lens. The minimum diameter of the optical spot formed by thismeans on the optical information recording medium is definedsubstantially as λ/NA by the wavelength λ of the laser beam and thenumerical aperture NA of the objective lens. In order to improve therecording density of the optical recording medium, on the other hand,the arrangement gaps (mark pitch) of the information marks in theoptical spot scanning direction may be reduced. When the mark pitchbecomes smaller than the spot diameter, however, the optical spotradiates simultaneously parts of other adjacent information marks whenit radiates a target information mark. Therefore, signals of theadjacent information marks leak to the signal of the information markthat is to be reproduced (this leak will be hereinafter referred to as“inter-symbol interference”). This interference interferes with noisecomponents and lowers reproduction accuracy. In a system that includes alaser having a specific wavelength and an objective lens, theinterference of the signals of the adjacent information marks renders acritical problem for achieving the high density.

[0006] A method that applies a wave form equalization processing to areproducing signal and reduces the inter-symbol interference has beenemployed in the past as means for lowering the mark pitch. Hereinafter,this equalization processing method will be explained with reference toFIG. 7 that schematically shows the wave form equalization processing. Areproducing signal 104 is inputted to an amplitude adjustment circuit500-1 and to a delay circuit 510-2. The amplitude adjustment circuit500-1 multiplies the reproducing signal 104 by a predetermined multiplein accordance with the equalization coefficient signal 502-1 outputtedfrom a coefficient generator 501. When the equalization coefficientsignal 502-1 is C1, for example, the reproducing signal 104 ismultiplied by Cl by a multiplication circuit 505-1 contained in theamplitude adjustment circuit 500-1, and is outputted as a signal-afteramplitude adjustment 520-1. On the other hand, the reproducing signal104 inputted to the delay circuit 510-2 is delayed by a predetermineddelay amount and is converted to a signal-after-delay 511-2. Theequalization processing comprises a plurality of processing as shown inFIG. 7, and is therefore executed serially. In consequence,signals-after-amplitude adjustment 520-1 to 520-n, each receiving anintrinsic delay amount and an intrinsic amplitude change, are acquired.These signals-after-amplitude adjustments 520-1 to 520-n are added by anaddition circuit 530 and a signal-after-equalization 108 is outputtedconsequently. If the equalization coefficient signal 502-1 to 502-noutputted from each coefficient generator 501-1 to 501-n is set inadvance to an appropriate value, the amount of the inter-symbolinterference contained in the signal-after-equalization 108 can bedrastically reduced. These equalization coefficients and delay amountsare set in most cases to optimum values that are determinedexperimentally. Incidentally, when n=3, the processing is referred to as“3-tap equalization processing” and when n=5, “5-tap equalizationprocessing”.

[0007] Incidentally, explanation of reference numerals 500-2, 502-2,500-n, 502-n, 510-n and 511-n will be omitted because it is the same asthe explanation of the reference numerals 500-1, 502-1, 510-2 and 511-2.

[0008] The diameter of the optical spot used for reproduction is definedsubstantially as λ/NA by the wavelength λ of the laser beam and thenumerical aperture NA of the objective lens, as described above. In thiscase, the highest temporal frequency that can be reproduced is (4×NA)/λ.As the frequency of the highest density repetition signal recordedapproaches the temporal frequency, the signal amplitude in reproductionbecomes smaller, and reproduction becomes more difficult. Therefore,when the high density is achieved by reducing the mark pitch, thehighest density repetition signal involves deterioration of asignal-to-noise ratio (S/N) resulting from the drop of the amplitude,and reproduction accuracy drops.

[0009] The amplitude of the highest density repetition signal can beincreased generally when the inter-symbol interference is reduced by theequalization processing described above. In consequence, the S/N can beimproved. However, when a higher density is attained by reducing furtherthe mark pitch, the wave form equalization system cannot acquire asufficient S/N improvement effect while reducing the inter-symbolinterference. The result is shown in FIGS. 6A-6E. FIGS. 6A-6E show thesimulation result of the reproducing signals in accordance with theHopkins' diffraction calculation described in “J. Opt. Soc. Am.”, Vol.69, No. 1, January (1979), pp 4-24, that executes the simulation of theoptical disk reproduction process in consideration of opticaldiffraction due to the information marks and the numerical aperture NAof the objective lens. This simulation assumes an 8-16 modulation systemusing a light source wavelength of 660 nm, an objective lens numericalaperture NA of 0.6, and a recording linear density on tracks of 28μm/bit. Since a window width (Tw) is 0.14 μm in this case, the highestdensity repetition signal is recorded as a repetition of a patterncomprising a recording mark having a length of 0.42 pm and anon-recorded portion having a length of 0.42 pm.

[0010]FIG. 6A shows the display of the eye pattern of the reproducingsignals before processing. It can be appreciated that opening cannot beobtained sufficiently in the proximity of the slice level (level “0”)due to the inter-symbol interference from the preceding and subsequentrecorded marks. FIG. 6B shows the eye pattern as a result of theequalization processing of this signal. The equalization processing uses3-tap equalization processing of n=3. The coefficients are set tod1=d3=−0.12 and d2=1.0 and the delay amount by the delay circuit istwice the window width. In consequence, the inter-symbol interferencecan be reduced. It can be appreciated that opening of the eye in theproximity of the slice level becomes greater than in FIG. 6A. However,the amplitude of the highest repetition signal is about ⅓ of theamplitude of the highest density repetition signal, and a sufficient SINcannot be obtained. FIG. 6C shows the eye pattern when the coefficientsare set to d1=d3=−0.30 and d2=1.0, and the amplitude of the highestdensity repetition signal is increased. In this case, the edge shiftbecomes great, and opening in the proximity of the slice level becomessmall, on the contrary, though the amplitude of the highest repetitionsignal becomes great. As described above, the wave form equalizationprocessing system cannot obtain a sufficient S/N improvement effectwhile reducing the inter-symbol interference, and the problemencountered in achieving the high density by reducing the mark pitchremains yet unsolved.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a methodcapable of increasing the amplitude of the highest density repetitionsignal while keeping the inter-symbol interference reduced and at thesame time, to provide an information reproducing apparatus foraccomplishing the method of the present invention.

[0012] The present invention scans information marks recorded in trackson a recording medium by a laser beam to generate a reproducing signal,and executes an equalization processing for reducing an inter-symbolinterference by serially changing equalization coefficients inaccordance with the level of the reproducing signal.

[0013] In order to have the present invention more easily understood,explanation is given first on the eye patterns.

[0014] In the eye pattern shown in FIG. 6C, the equalization processingis executed using equalization coefficients that have a large absolutevalue in order to reduce the inter-symbol interference generated by thecontinuation of short marks and short non-recorded portions, and toincrease the amplitude of the highest density repetition signal. Atthose portions in which long marks and long non-recorded portions exist,however, the reduction of the inter-symbol interference is attemptedalthough the inter-symbol interference does not exist there from theoutset. Therefore, distortion develops in the wave form with the resultthat the edge shift occurs. In other words, the equalization coefficientfor reducing the edge shift is different between the highest densityrepetition signal and other signals. For this reason, the equalizationprocessing using the equalization coefficient that is kept fixed at apredetermined value cannot increase the amplitude of the highest densityrepetition signal while keeping the inter-symbol interference at a lowlevel.

[0015] The present invention executes the equalization processing usingan appropriate equalization coefficient for each signal. Hereinafter,the present system will be explained. FIG. 5 shows the relation betweenthe absolute value of the reproducing signal and the equalizationcoefficient. The present system gives an appropriate equalizationcoefficient to each signal on the basis of this relation. The smallerthe absolute value of the reproducing signal, the greater becomes theequalization coefficient. When the absolute value is 0, the equalizationcoefficient is a. As the absolute value of the reproducing signalbecomes great, the equalization coefficient becomes small. When theabsolute value of the reproducing signal attains the maximum value 1,the equalization coefficient becomes b.

[0016] When a short mark is reproduced, the absolute value of theresulting reproducing signal becomes small. When a long mark isreproduced, on the contrary, the absolute value of the resultingreproducing signal is great. In other words, according to the ruledepicted in FIG. 5, a large equalization coefficient is used for a shortmark. Therefore, the inter-symbol interference can be greatly reducedand the amplitude after equalization becomes greater than before. Asmall equalization coefficient is used for a long mark. Therefore, theamplitude after equalization does not much change than beforeequalization. This also holds true of the edge shift. The equalizationis positively executed near the portions where the absolute value issmall, that is, the portions where the inter-symbol interference islikely to occur because the mark length and the length of thenon-recorded portions are small. On the other hand, the equalization ishardly executed near the portions where the absolute value is great,that is, near the portions where the inter-symbol interference isdifficult to occur because the mark length and the length of thenon-recorded portions are large. As a result, the waveform distortionresulting from excessive equalization can be eliminated, in principle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a block diagram showing an optical informationreproducing apparatus according to one embodiment of the presentinvention;

[0018]FIG. 2 is a circuit diagram showing an example of a non-linearequalization circuit that constitutes the optical informationreproducing apparatus according to the present invention;

[0019]FIG. 3 is a circuit diagram showing another example of thenon-linear equalization circuit that constitutes the optical informationreproducing apparatus according to the present invention;

[0020]FIG. 4 is a circuit diagram showing still another example of thenon-linear equalization circuit that constitutes the optical informationreproducing apparatus according to the present invention;

[0021]FIG. 5 is a graph useful for explaining calculation of anequalization coefficient in the present invention;

[0022] FIGS. 6A-6E are eye pattern comparison diagrams useful forexplaining the features of the present invention; and

[0023]FIG. 7 is a circuit diagram showing an equalization circuit usefulfor explaining the related art.

DESCRIPTION OF THE EMBODIMENTS

[0024] Preferred embodiments of the present invention will be explainedhereinafter with reference to the accompanying drawings. To begin with,an equalization circuit will be explained as one of the features of thepresent invention.

[0025]FIG. 3 is a circuit diagram showing an example of a non-linearequalization circuit 107 according to the present invention. Amplitudeadjustment circuits 160-01 to 160-03 adjust the amplitude of areproducing signal 106 in accordance with the rule shown in FIG. 5. As aresult, a portion having a small amplitude is adjusted to a largeamplitude and is outputted. A portion having a large amplitude isconverted to a small amplitude and is outputted. Coefficient generators210-1 to 210-3 give the maximum value a shown in FIG. 5 of coefficientsa1 to a3 and give the minimum value b shown in FIG. 5 among thecoefficients b1 to b3. The coefficient generators 210-1 to 210-3 outputalso coefficients c1 to c3 that determine the polarity of the signalthat is subjected to amplitude adjustment. FIG. 6D shows the eye patternwhen the coefficients a1, b1 and c1 are set to 0.4, 0.1 and −1,respectively, the coefficients a2, b2 and c2 are set to 1.0, 1.0 and +1,respectively and the coefficients a3, b3 and c3 are set to 0.4, 0.1 and−1, respectively, and the delay amount by delay circuits 150-2 to 150-3is twice the detection window width. It can be seen that in comparisonwith the eye pattern obtained by the equalization processing shown inFIG. 6B, the edge shift amount remains unaltered but the amplitude ofthe highest density repetition signal is improved more greatly.

[0026] Incidentally, reference numerals 161-1, 161-2 and 161-3 denotesignals-after-amplitude adjustment. Reference numeral 151-2 and 151-3denote signals-after-delay. Reference numerals 211-1, 211-2 and 211-3denote the maximum value signals of the coefficients, and referencenumerals 212-1, 212-2 and 212-3 denote the minimum value signals of thecoefficients. Reference numerals 213-1, 213-2 and 213-3 denote polaritysignals. Reference numeral 170 denotes an addition circuit and referencenumeral 108 denotes a signal-after-equalization.

[0027]FIG. 4 shows another example of the non-linear equalizationcircuit 107 according to the present invention. The amplitude adjustmentcircuits 160-1 to 160-5 and the coefficient generators 210-1 to 210-5function in exactly the same way as the corresponding ones shown in FIG.3, respectively. FIG. 6E shows the eye pattern when the coefficients a1,b1 and c1 are set to 1.0, 0.3 and −1, respectively, the coefficients a2,b2 and c2 are set to 1.0, 0.8 and +1, respectively, the coefficients a3,b3 and c3 are set to 1.0, 1.0 and +1, respectively, the coefficients a4,b4 and c4 are set to 1.0, 0.3 and +1, respectively, and the coefficientsa5, b5 and c5 are set to 1.0, 0.3 and −1, respectively, and the delayamount by the delay circuits 150-2 to 150-5 is set to the same amount asthe detection window width. It can be seen that the amplitude of thehighest density repetition signal is improved most greatly in the eyepatterns shown in FIGS. 6A-6E.

[0028] Incidentally, reference numerals 161-1 to 161-5 denotesignals-after-amplitude adjustment. Reference numerals 151-2 to 151-5denote signals-after-delay. Reference numerals 211-1 to 211-5 denote themaximum values of the coefficients and reference numerals 212-1 to 212-5denote the minimum values of the coefficients. Reference numerals 213-1to 213-5 denote polarity signals. Reference numeral 106 denotes areproduction signal. Reference numeral 170 denotes an addition circuitand reference numeral 108 denotes a signal-after-equalization.

[0029] As described above, when the equalization processing according tothe present invention is employed, the amplitude of the highest densityrepetition signal can be improved without increasing the edge shift.Because the S/N can be improved eventually, the high density can beachieved by decreasing the mark pitch.

[0030] Next, an information reproducing apparatus using the non-linearequalization circuit 107 of the present invention will be explained withreference to FIG. 1. FIG. 1 is a schematic structural view of theoptical information reproducing apparatus according to the presentinvention.

[0031] This optical information reproducing apparatus comprises anoptical recording medium 100 that is mounted to a driving device 101 androtated, an optical head 102 that radiates an optical spot 103 to theoptical recording medium 100 and reproduces the recorded information, alaser driving circuit 115 that drives the optical head 102 so that a rayof light having predetermined optical power can be outputted from theoptical head 102, a servo circuit 113 that positions the optical spot103, an automatic gain control circuit 105 that adjusts the amplitude ofthe reproducing signal to a predetermined amplitude on the basis of thereproducing signal 104 obtained when the optical spot 103 scans thetracks, a non-linear equalization circuit 107 that executes a signalprocessing for reducing inter-symbol interference by serially changingthe equalization coefficients in accordance with the level of thereproducing signal the gain of which is adjusted, a phase locked loopcircuit 109 that generates a clock signal in synchronism with therevolution of the optical recording medium 100 on the basis of thereproducing signal that is subjected to the equalization processing, adata demodulation circuit ill that demodulates the reproducing signalsubjected to the equalization processing in accordance with apredetermined demodulation system, and outputs a user data, and acontrol circuit 117 that controls the servo circuit 113, the laserdriving circuit 115 and the data demodulation circuit 111.

[0032] Reference numeral 108 denotes a signal-after-equalization.Reference numeral 110 denotes a sync signal. Reference numeral 112denotes output data. Reference numeral 114 denotes a servo signal to beapplied to the optical head 102. Reference numeral 116 denotes a laserdriving signal for generating the optical spot 103 from the optical head102. Reference numeral 118 denotes a control signal to be applied fromthe control circuit 117 to the servo circuit 113. Reference numeral 119denotes a control signal to be applied from the control circuit 117 tothe laser driving circuit 115. Reference numeral 120 denotes a controlsignal to be applied from the control circuit 117 to the datademodulation circuit 111.

[0033] The non-linear equalization processing 107 shown in FIG. 1 willbe further explained. FIG. 2 shows another example of the non-linearequalization circuit according to the present invention. This circuitcomprises a plurality of amplitude adjustment circuits 160-1 to 160-n, aplurality of coefficient generators 210-1 to 210-n, a plurality of delaycircuits 150-2 to 150-n, and one addition circuit 170, in the same wayas the non-linear equalization circuit 107 that is explained withreference to FIGS. 3 and 4. The amplitude adjustment circuit 160-1comprises an absolute value circuit 180-1, multiplication circuits 190-1and 200-1, a polarity circuit 220-1 and a delay circuit 230-1.

[0034] The reproducing signal-after-gain control 106, that is inputtedto the non-linear equalization circuit 107, exhibits the eye patternsuch as the one shown in FIG. 6A. The signal is subjected to theamplitude adjustment by the automatic gain control circuit 105 of thepre-stage so that the maximum value of the amplitude and its minimumvalue reach the predetermined values. In the example shown in FIG. 6A,the amplitude adjustment is made so that the maximum value is +1 and theminimum value is −1. It will be assumed hereinafter for convenience sakethat the automatic gain control circuit 105 adjusts the amplitude sothat the maximum value of the reproducing signal 104 is +1 with itsminimum value being −1, and outputs the reproducing signal 106 aftergain control.

[0035] As shown in FIG. 2, the absolute value circuit 180-1 generatesthe absolute value of this reproducing signal 106 after gain control andoutputs an absolute value signal 181-1.

[0036] The multiplication circuit 190-1 outputs the equalizationcoefficient 191-1 corresponding to the absolute value of the reproducingsignal 106 on the basis of the maximum value signal 211-1 of theequalization coefficient signal and its minimum value signal 212-1 thatare given by the coefficient generator 210-1. The rule for calculatingthe equalization coefficient z from the absolute value |X| of thereproducing signal 106 may use the relationship shown in FIG. 5, forexample. The value z is expressed as z=(b−a)×|X|+a by the arithmeticexpression. Here, the maximum value of the equalization coefficientsignal is a and its minimum value is b. The multiplication circuit 190-1has the function of outputting z=(x1−x2) (y1−y2)+U to the inputterminals x1, x2, y1, y2 and U. Therefore, the rule shown in FIG. 5 canbe accomplished circuit-wise by inputting the absolute value signal181-1 to the input terminal x1, 0 to the input terminal x2, the minimumvalue signal 212-1 of the equalization coefficient signal to the inputterminal y1, and the maximum value signal 211-1 of the equalizationcoefficient signal to the input terminals y2 and U.

[0037] The polarity circuit 220-1 outputs the equalization coefficient191-1 as the equalization coefficient with polarity 221-1 when thepolarity signal 213-1 given by the coefficient generator 210-1 is +1,and outputs the equalization coefficient 191-1, the polarity of which isinverted, as the equalization coefficient with polarity 221-1 when thepolarity signal 213-1 is −1.

[0038] The delay circuit 230-1 has the delay amount equal to thepropagation delay amount of the signal that occurs in the absolute valuecircuit 180-1, the multiplication circuit 190-1 and the polarity circuit220-1, and functions in such a fashion that the signal-after-phasecompensation 231-1, that adds the delay to the reproducingsignal-after-gain control, and the equalization coefficient withpolarity 221-1 have the same phase.

[0039] The multiplication circuit 200-1 multiplies the issignal-after-phase compensation 231-1 by the equalization coefficientwith polarity 221-1 and outputs the product as thesignal-after-amplitude adjustment 161-1.

[0040] On the other hand, the reproducing signal-after amplitudeadjustment 106 is delayed by the delay amount set in advance by thedelay circuit 150-2 and is converted to the reproducingsignal-after-delay and gain adjustment 151-2. This signal is processedby the amplitude adjustment circuit 160-2 in the same way as describedabove, and the signal-after-amplitude adjustment 161-2 is generated.Therefore, each of the signals 161-1 to 161-n generated by eachamplitude adjustment circuit is the signal to which an intrinsic delayamount and intrinsic amplitude adjustment are imparted.

[0041] The addition circuit 170 adds all of thesesignals-after-amplitude adjustment 161-1 to 161-n and outputs thesignal-after-equalization 108. Incidentally, the values of thecoefficient generators 210-1 to 210-n and the values of the delaycircuit 150-2 to 150-n are set in advance so that the inter-symbolinterference contained in the signal-after-equalization 108 becomesminimal.

[0042] Incidentally, reference numerals 211-2 and 211-n denote themaximum value signals of the coefficient, reference numeral 212-2 and212-n denote the minimum value signals of the coefficient and referencenumerals 213-2 and 213-n denote the polarity signals.

[0043]FIG. 3 shows an example of a 3-tap equalization circuit accordingto the present system. FIG. 6D shows the eye pattern when thecoefficients a1 b1 and c1 in FIG. 3 are set to 0.4, 0.1 and −,respectively, the coefficients a2, b2 and c2 are set to 1.0, 1.0 and +1,respectively, and the coefficients a3, b3 and c3 are set to 0.4, 0.1 and−1, respectively, and the delay amount by the delay circuit is twice thedetection window width. It can be seen that in comparison with the eyepattern obtained by the conventional equalization processing shown inFIG. 6B, the amplitude of the highest density repetition signal can bedrastically improved though the edge shift quantity remains unaltered.

[0044] Incidentally, the absolute value circuit 181-1 shown in FIG. 2determines the absolute value |X| as the size of the reproducing signal.The difference is likely to occur as a mean DC level in the case wherethe detection window width is the highest density reproducing signal(3Tw) or the lowest density reproducing signal (11Tw), if the mean DClevel of the highest density reproducing signal (eTw) is an asymmetry X₀as positioned at a central or maximum amplitude, but the eye patternshown in FIG. 6E can be obtained when the absolute value circuit 181-1is so constituted as to determine the absolute value |x−x₀|.

[0045]FIG. 4 shows an example of a 5-tap non-linear equalization circuitaccording to the present system. FIG. 6E shows the eye pattern when thecoefficients a1, b1 and 1 in FIG. 4 are set to 1.0, 0.3 and −1,respectively, the coefficients a2, b2 and c2 are set to 1.0, 0.3 and +1,respectively, the coefficients a3, b3 and c3 are set to 1.0, 1.0 and +1,respectively, the coefficients a4, b4 and c4 are set to 1.0, 0.3 and +1,and the coefficients a2, b2 and c2 are set to 1.0, 0.3 and −1,respectively, and the delay amount by the delay circuit is equal to thedetection window width. It can be seen that the amplitude of the highestrepetition signal is improved most greatly in each eye pattern shown inFIGS. 6A-6E.

[0046] As described above, the amplitude of the highest densityrepetition signal can be increased according to the equalizationprocessing of the present invention without increasing the edge shift.Because the S/N can be eventually improved, high reliability can besecured and the high density can be achieved even when the mark pitch isreduced.

[0047] The information reproducing apparatus explained in the foregoingembodiments can be applied to both the analog system and the digitalsystem.

[0048] The present invention applies the signal processing, that reducesthe inter-symbol interference by serially changing the equalizationcoefficients in accordance with the level of the reproducing signals, tothe reproducing signals, and can increase the amplitude of the highestdensity repetition signal without increasing the edge shift. Therefore,since the S/N can be improved, high reliability can be secured even whenthe mark pitch is decreased, and the high density can be achieved.Additionally, the equalization processing according to the presentinvention can be applied to not only the phase-change optical disk butalso the opto-magnetic disk.

What is claimed is:
 1. A method of reproducing information by scanninginformation marks, disposed with a predetermined mark pitch therebetweenand recorded on tracks inside a predetermined information recordingregion, by an optical spot, detecting an optical change of saidinformation marks by said scanning operation, applying signal processingto a reproducing signal obtained by said detection operation so as toreduce inter-symbol interference of an interference amount varying fromone of said information marks to another of said information marks, andreproducing the information corresponding to said information mark,comprising the steps of: a) detecting the size of the optical changeresulting from said information mark; b) calculating a plurality ofequalization coefficients used for an equalization processing for thesize of said optical change detected for each of said information marks;and c) reducing said inter-symbol interference on the basis of saidequalization coefficients by said equalization processing; wherein saidequalization coefficient used for said equalization processing of thesize of said optical change by a first information mark of a firstlength to reduce said inter-symbol interference in said step c) isgreater than said equalization coefficient used for said equalizationprocessing of the size of said optical change by a second informationmark of a second length which is greater than said first informationmark.